Method for degassing molten metal



Feb, 22, 1966 c. w. FlNKL 3,236,635

METHOD FOR DEGASSING MOLTEN METAL Filed Dec; 2, 1958 3 Sheets-Sheet 1,HHH fl IN VEN TOR.

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Feb. 22, 1966 c. w. FINKL 3,236,635

METHOD FOR DEGASSING MOLTEN METAL Filed Dec. 2, 1958 3 Sheets-Sheet 2 7INVENTOR. J 2 Kim/es IV. @7714,

United States Patent 3,236,635 METHOD FOR DEGASSING MOLTEN METAL CharlesW. Finkl,-Chicago, Ill., assignor to A. Finkl & Sons Company, Chicago,Ill., a corporation of Illinois Filed Dec. 2, 1958, Ser. No. 777,664 23Claims. (Cl. 7549) My invention relates to improvements in method andapparatus for degassing molten metal. It is especially aimed at loweringthe hydrogen, nitrogen, and oxygen content, whether in solution or inphysical mixture therein, of the metal. These gases, though small inweight in proportion to the mass of metal, are very deleterious.

One example of this invention is to reduce the hydrogen content ofmolten steel by purging with an inert gas for example, helium or argon,under vacuum conditions. Excess hydrogen promotes flaking and hydrogenembrittlement. These hazards are presently compensated for by heattreatment, but by reducing the hydrogen content, the heat treating timeis materially reduced, which in turn reduces cost, and a better overallproduct results. In addition, since the limiting production factor inquantity runs is often the isothermal annealing, the annealing capacityof the plant is eifectively increased.

It is old to subject a ladle of steel to a vacuum, thus drawing oh. someof the hydrogen from the molten metal. Unfortunately almost invariablyStratification occurs and while substantial removal of hydrogen from theupper strata can be accomplished, the entire mass is not adequatelysubjected to the vacuum.

Another proposed solution has been to bubble an inert gas upwardlythrough the ladle of molten metal. Though claims to much better resultshave been made, as a practical matter it is seldom possible to reducethe hydrogen content down to a figure smaller to that attained with avacuum alone.

What is proposed is to place the usual conventional type of ladle in avacuum chamber, whereby air can be exhausted from the chamber and gasintroduced into the bath, thus simultaneously subjecting the surface ofthe ladle to a vacuum while bubbling the gas through the ladle from thebottom. Under these circumstances, what occurs is that the bubbles ofgas travel upwardly through the molten mass, stirring and agitating itand greatly increasing the surface of the liquid exposed to the gas, thesurface in this case being not merely the surface of the level of theliquid at the top of the ladle but the combined areas of all theindividual gas bubbles as they travel up from the bottom of the ladle toand are discharged through the level of the liquid metal. The hydrogenin the surfaces exposed to these bubbles diffuses into the bubbles andis discharged from the vacuum chamber by the vacuum pump. The gases inthe molten metal are usually evolved as hydrogen, nitrogen, and carbonmonoxide, but may be evolved in other combinations.

In many applications, this upward travel of the bubbles sets up acurrent so that there tends to be a circulation of the molten metalupwardly adjacent the center of the ladle and downwardly from thesurface around the outer periphery. This agitation insures that amaximum proportion of the molten metal is exposed to the gas and3,236,635 Patented Feb. 22, 1966 has the opportunity to give up thedeleterious hydrogen or other gases in the metal.

The combination of bubbling a gas into the metal while subjecting thesurface of the metal to a vacuum greatly increases unexpectedly thedegree of removal of the undesired hydrogen or other gas and makes ispossible to carry the removal down far below that with either of theother two previously known methods.

The exact amount of bubbled gas needed and the pressures and degrees ofvacuum may vary, but in general, it is essential that the bubbled gas beinjected into and discharged close to the bottom of the molten metal ata pressure sufficiently above the static head of the metal to insureescape and upward bubbling of the gas. The type of gas bubbled throughthe melt is not restricted to inert gases, because even dry air may beused. It is only essential that the gas be one that will not combine tooreadily with the melt.

My invention is illustrated more or less diagrammatically in theaccompanying drawings, wherein FIGURE 1 is a plan view of the device;

FIGURE 2 is a section along the line 2--2 of FIG- URE 1;

FIGURE 3 is a section along the line 3-3 of FIG- URE 1;

FIGURE 4 is an elevation with parts in section of a modification; and

FIGURE 5 is a modification of an inspection window assembly.

Like parts are indicated by like characters throughout the specificationand drawings.

Foundation beams 1 support a platform 2 from which rises a metal tank 3.A supporting ring 4 rises from the platform 2 and terminates at the topin a bearing ring 5. Insulation 6 protects the platform 2. The upperboundary of the tank 3 terminates in a channeled flange 7 adapted toreceive a sealing ring 8. A dome 9 is carried by the pivoted arm 10 oncolumn 11 and rests on the sealing ring 8. The hydraulic ram 12 isinterposed between the center of the dome which is concentric with thetank and the arm 10 so that the dome may be raised and lowered into andout of register with the tank, the tank and dome being both circular.The dome is bounded at its outer lower periphery by a reinforcing flange13 in opposition to the ring 8. Aligning fingers 14 on the flange 13assist in centering the dome on the tank so that when the dome isseated, the seal ring 8 provides a gas tight closure between the domeand tank.

A heat shield 15 which is apertured at 16 is supported by brackets 17from the dome. Barrel 18 extends from the dome about the aperture is andis in register with the aperture 16 in the shield 15. The contents ofthe ladle in the vacuum tank may be inspected through an inspectionwindow 20.

A pouring ladle 21, which is lined as at 22, is open at the top andprovided with a flange ring 23 which may v rest on the bearing ring 5.The flange ring 23 is provided at 24 with aligning guide elementsassociated with the A gas injection pipe 26, which is open at the lowerend and surrounded by insulating sleeve members 27, extends downwardlythrough an aperture 28 in the dome and aperture 29 in the shield and maybe raised and lowered by any suitable hoist mechanism, the details ofwhich are not here shown. A seal 30 is carried by the upper end of thepipe 26, and so disposed that when the pipe is in its lower position,the seal closes the port 28 and makes a gas tight joint between the pipe26 and the dome 9. A gas conduit 31 provides a carrier agent or gas fromany suitable source, the details of which are not here shown.

At the lower part of the tank adjacent one side, is an aperture 32 inregister with a gas exhaust pipe 33 which leads to a suitable vacuumconnection. A baffle 34 extends inwardly from the wall of the tank 3, tomask the opening 32 to prevent heavy particles from entering the vacuumsystem. A concrete foundation pit 35 encloses the lower part of thestructure.

FIGURE 3 illustrates a safety arrangement. The mirror 36 in the pit canbe seen from above the pit and reflects the window 37 through which theoperator may check the bottom of the tank. If there should be anyleakage, for instance through the stopper nozzle 38 or otherwise, theoperator will see that so that the ladle can be immediately withdrawn.

The insulated gas injection pipe is lighter than the solid molten metalso the weight 39 may be added to the pipe to give it weight sufiicientto overcome specific gravity of the molten metal and remain inpenetrated gas injection position.

Under some circumstances it may be desirable to inject the gas throughthe bottom of the ladle, the result being substantially the same.

An alternate structure for practicing this method is illustrated inFIGURE 4. Ladle 40 is provided with a vertical lining 42. The bottom iscomposed of a double layer construction of refractory material 44 and46. Upper layer 44 may be composed of a plurality of individual sectionswhich when assembled provide an aperture 48 near the center of thebottom. Alternately, the aperture may be formed in a single slab oflining.

A diffusing plug, consisting of an upper portion 50 and a lower portion52, is located in aperture 48. Gas injection pipe 54 projects upwardlythrough the bottom of the ladle and terminates in the body of the lowerportion of the plug. This portion is composed of a porous refractorymaterial so that the carrier agent discharged from the injection pipewill pass upwardly. The upper portion may similarly be composed of aporous refractory material or it may be a removable metallic nozzle.

The injection pipe is connected to a gas conduit 56 which extendsupwardly along the outside of the ladle and is connected to suitablevalving on the exterior of the tank enclosing the ladle. The particularvalving structure is not essential and accordingly has not beenillustrated. The conduit may be connected by a flexible hose to a tankor other source of gas under pressure.

A gas bottle 60 secured to the ladle by clamps 62 and 64 containingbubbling gas under pressure is supported on a ledge 66 extending aboutthe exterior of the ladle. Pressure regulator 68 maintains asubstantially uniform pressure into an orifice 70. Suitable regulatorycontrols may be provided to control the flow rate of gas. If tank 60contains less than the amount of gas needed to completely bubble a heat,the additional supply required may be furnished by the exterior source.

Bubbling a gas through a ladle under vacuum may cause a boil so violentthat drops of the metal may splash into the sight glass structure andcompletely obscure the glass. In FIGURE 5, an inspection window assemblyis shown which may be utilized in place of the unobstructed tube 18 ofFIGURE 2. The assembly includes an outer housing 74 open at one end 76and closed by a glass retainer wall 78 at the opposite end. End 76 hasbeen 4 formed at a substantial angle in order to provide a convenientangle of sight into the ladle. A Window glass 80 and seal 82 arereceived in the glass retainer wall 78. Abutment plate 84 maintains atight engagement between the glass and wall.

An apertured plate 86 is positioned near the mid= portion of the casingand held in place by a key and slot arrangement 88 and rolled angle 90.The center line 92 of aperture 94 is located slightly below the centerline 96 of the sight glass so that lines of sight 98 and 100 reproducethe open area at end 76 at the glass 80. A wiper 102 having a projectinghandle 104 may be utilized to sweep the inside of the sight glass. Theaperture plate prevents spattering of the sight glass by materiallyreducing the area exposed to the boiling metal.

The use and operation of my invention are as follows:

The molten metal, which may for example be a ferrous alloy, is pouredinto the ladle in the usual way. Meanwhile the dome has been swung asideto leave the vacuum tank open. The crane lowers the ladle into thevacuum tank. At the same time vacuum commences to be drawn from thetank, the crane hooks are disconnected, the ladle being seated in theposition shown in FIGURE 2. Then the dome is swung into register withthe tank and is lowered to seal the tank about the mating peripheries ofthe tank and dome. Then the pipe 26 is lowered through the dome andshields until it reaches a point a little above, perhaps three inches orso, the bottom of the ladle and the aperture through which the pipe waslowered, is sealed. Vacuum continues to be drawn from the tank. As soonas a seal is accomplished, a carrier agent or gas, for example-heliumthough argon or other gases may be usedis forced into the pipe at apressure above the ferrostatic pressure of the molten iron in the ladleand commences to bubble up through the liquid, being drawn out as aresult of the vacuum drawn on the tank.

The vacuum during some stages of the process may be one millimeter ofmercury or less. Several minutes will usually be required to take thevacuum down to that point depending on such factors as the capacity ofthe vacuum system, the size of the vacuum container and piping, the typeof steel under treatment, and the size of the ladle. The purging gas isthen forced into the metal, the time depending upon the size of theheat, type of metal under treatment, depth of ladle, and other factors.After the gas is turned off, the vacuum system is valved off and aneutral gas such as nitrogen is introduced into the chamber in order toguard against the formation of an explosive atmosphere. Air is thenadmitted to bring the pressure within the chamber up to atmospheric. Inthe embodiment of FIGURE 2, the gas pipe is drawn out of the ladle, ifused, and the dome is raised and swung aside and the ladle withdrawn.

Just as soon as the ladle is placed, vacuum can be started and just assoon as the pipe is inserted into the ladle or the gas control valves ofFIGURE 4 are opened, the gas will start to flow. As above indicated, thegas pressure must be sufficient to overcome the static head of themetal. The static head of the metal is one atmosphere for approximatelyeach five feet of depth but as the pressure in the tank falls less andless gas pressure is needed to insure operation. One convenient way ofcontrolling the flow of gas is to regulate gas pressure to twenty poundsper square inch, thereafter gas will pass through a variable orificeflowmeter. Pressure downstream will vary in consonance with the pressurerequired to insure a continuous flow of the gas at the desired rate toremove the particular amount of hydrogen in the liquid metal.

A typical example of the results obtained with this process as comparedto the present processes utilizing only a vacuum or a gas individuallyis described in the accompanying table. These results are relative, andeven lower results might be obtained under different conditions in whichonly a vacuum or a purging gas are utilized. In

these heats, a sixty-ton ladle containing a medium carbon steel having aChrome-Nickel-Moly analysis of approximately .90, 1.0, and .30respectively was utilized. The ladle was placed in a Vacuum systemdefining approximately 1200 cubic feet having a four stage steam ejectorpump system.

N0. 1, No. 2, No. 3, Bubbled Vacuum Bubbled Only Only and Vacuum HeatDescription:

Size of Heat, Tons 36 35 35 Gas Bubbled Helium None Helium Flow Rate,c.f.h 300 300 Amount Gas Bubbled, Cu.

NO. 1, H2 N0. 2, 112 N0. 3, 112

Gas Analysis Results:

Electric Furnace Before Tap- 4. 9 6 5 5. 4 Ladle Surface Before Process5. 1 7 3 5. 3 Ladle Surface After Process- 4. 9 5 3 4. 2 Metal FromLadle Stream After 14 Tons Have Been Poured 4. 8 5. 5 3.1 Metal FromLadle Stream After 31 Tons Have Been Poured 4. 9 5. 5 2. 7

Time in Tank, Min.:

Under some conditions, a unique phenomenon characterized by a violentboil occurs. Just a few minutes after a vacuum of one millimeter isreached, the absolute pressure in the tank will rise sharply and themetal will boil so violently so as to almost overflow the ladle. Theboil can then be controlled by dropping the initial stages of the vacuumuntil the boil subsides to a fiat bath. The best results are generallyobtained when this condition occurs. Although the exact physical andchemical changes have not been ascertained it is thought that the boilmay result from a disassociation of oxides and nitrides.

The foregoing tabular results show that the combined use of the vacuumand purging yields a hydrogen content of only slightly over 50% of thebest results obtainable by prior methods.

Although lower absolute values might be obtained by the use of eitherpurging or vacuum alone under different conditions with a particularsteel, the simultaneous use of vacuum and a purging gas invariablyproduces better results.

The gas injection tube at the center of the ladle insures that as thegas bubbles up, it travels along a path gen erally axial with respect tothe ladle entraining with it molten metal. This molten metal will flowvertically, then radially, along the surface and will tend to migratedownwardly along the outer periphery of the ladle, such migration beingalso promoted by the fact that cooling of the metal is from the ladlewalls inwardly. The bubbling accomplishes two things. First, theindividual bubbles act as carrier agents to remove some included gas andsecondly the agitation induced moves virgin metal from the bottom to thetop of the ladle where it may be subjected to the vacuum. The vacuum iseffective to a depth of a few inches to a few feet depending on boil.

As mentioned, a variety of factors must be taken into account, butperhaps the most important ones are the analysis of the steel, the depthof the ladle, and the meth- 0d of analyzation. It has been found thatincluded gas diffusion into the purging gas, and perhaps into CO, variesin a geometrical ratio to the ladle depth. With the equipment utilizedin the exemplary heats, the rate of diffusion varied approximatelyproportionally to the square of the depth. The method of analyzationincludes such factors as where the sample is taken, i.e. whether in amolten condition or from the finished product, how taken, i.e. by a pintube, an evacuated copper cylinder, or a core drill from the finishedproduct, and the equipment used in running the hydrogen contentanalysis. In the exemplary heats, evacuated pin tubes were used to takethe sample from a bare ladle spoon, and a Fisher Serfass Fusion GasAnalyzer was used to run the sample.

The amount of slag present and the addition of aluminum will also affectthe results. When the slag forms a continuous blanket over the surface,the bubbling action is reduced, and the vacuum is less effective. Theaddition of aluminum ties up the oxygen which prevents formation of CO.The CO also acts as a purging gas into which the hydrogen can diffuse.

Although only a single gas source near the bottom center of the ladlehas been described, any convenient number of pipes or plugs located insuitable positions may be utilized. The number and position will dependon such factors as the size of ladle.

I claim:

1. A method of removing deleterious gases from a con fined volume ofmolten metal in a receptacle, said method including the steps ofsubjecting the surface of the confined volume of molten metal to avacuum suflicient to degas the molten metal, and simultaneously passinga sufficient quantity of a carrier agent upwardly through the moltenmetal to induce a circulation entirely within the receptacle whichbrings substantially undegassed molten metal from remote areas in thereceptacle to the surface.

2. The method of claim 1 further characterized in that the carrier agentis a gas having little afiinity for the molten metal.

3. The method of claim 2 further characterized in that the carrier agentis an inert gas.

4. The method of claim 1 further characterized in that the carrier agentis a gas selected from the group consisting of argon, helium, and dryair.

5. The method of claim 1 further characterized in that the carrier agentis passed upwardly through the molten metal from a point close to thebottomof the receptacle.

6. A method of degassing a batch of molten metal in a ladle, said methodincluding the steps of subjecting the surface of the molten metal to avacuum suflicient to degas the molten metal, and simultaneously bubblinga sulficient volume of purging gas having little afiinity for the moltenmetal upwardly through the molten metal to induce a circulation withinthe ladle which brings substantially undegassed molten metal from remoteareas in the ladle to the surface.

7. The method of claim 6 further characterized in that the purging gasis an inert gas.

8. The method of claim 6 further characterized in that the purging gasis bubbled upwardly through the bottom of the ladle.

9 The method of claim 6 further characterized in that the purging gas isbubbled upwardly through the molten metal from a point slightly abovethe bottom of the ladle.

10. A method of removing undesired gases from a batch of molten metal ina ladle, said method including the steps of subjecting the surface ofthe molten metal to a vacuum sufficient to degas the molten metal, andsimultaneously bubbling a sufiicient volume of purging gas having littleafiinity for the molten metal upwardly through the molten metal atapproximately the center of the ladle to thereby induce an upwardcirculation adjacent the center and a downward circulation adjacent theperiphery of the ladle which brings substantially undegassed moltenmetal from remote areas in the ladle to the surface.

11. A batch method of removing undesired gases in a batch of moltenmetal in a ladle, said method including the steps of subjecting thesurface of the molten metal to a vacuum on the order of about 1millimeter of mercury or less, and simultaneously bubbling a sufficientvolume of purging gas having little affinity for the molten metalupwardly through the molten metal to thereby induce a circulation withinthe ladle which brings substantially undegassed molten metal from remoteareas in the ladle to the surface.

12. A batch method of removing deleterious gases from a batch of moltenmetal in a receptacle, said method including the steps of subjecting thesurface of the batch of molten metal to a vacuum sufficient to degas themolten metal in the absence of a slag blanket of a thickness whichprevents the metal from being effectively exposed to the vacuum, andsimultaneously agitating the molten metal to thereby set up acirculation entirely within the receptacle which exposes remotesubstantially undegassed portions of molten metal directly to the vacuumat the surface of the batch, and adding an inert agent to thereby reducethe danger of explosion.

13. A method of degassing a batch of molten metal in a receptacle, saidmethod including the steps of placing molten metal in the receptacle,subjecting the surface of the molten metal to a vacuum sufficient todegas the molten metal, simultaneously exposing substantially undegassedmolten metal from remote areas in the receptacle directly to the vacuumby passing a sufficient volume of a carrier agent upwardly through themolten metal to induce a circulation within the receptacle which bringssubstantially undegassed molten metal from remote areas in thereceptacle to the surface, flooding the area above the surface of themolten metal in the receptacle with an inert agent to thereby reduce thedanger of explosion, and thereafter exposing the receptacle toatmospheric conditions.

14. A batch method of degassing molten metal in a ladle, said methodincluding the steps of tapping molten metal into a ladle, subjecting themolten metal to a vacuum sufficient to degas the molten metal,simultaneously bubbling a suflicient volume of purging gas at a pressureabove the head of the metal upwardly through the molten metal to inducea circulation entirely within the ladle which brings substantiallyundegassed molten metal from remote areas in the ladle to the surface,flooding the area above the surface of the molten metal in the ladlewith an inert gas to thereby reduce the danger of explosion, and thenexposing the ladle to atmospheric conditions.

15. In a process of removing deleterious gases by a vacuum purgingtreatment from a batch of molten steel having a depth greater than thedepth to which the vacuum alone is effective, the steps comprisingsubjecting the surface of the batch to a vacuum sufficiently low toeffectively degas it, and, simultaneously with subjection of the steelto the vacuum, passing a gaseous purging agent which does not combinewith or migrate into the steel upwardly through the batch to therebycause substantially undegassed molten steel in areas of the batch remotefrom the surface to be exposed to the vacuum at the surface, thesubjection of the surface of the batch to the vacuum occurring in theabsence of a slag blanket thereon which prevents the boiling metal frombeing effectively exposed to the vacuum.

16. The process of claim 15 further characterized in that the gaseouspurging agent is dry air.

17. The process of claim 15 further characterized in that the gaseouspurging agent is an inert gas.

18. The process of claim 15 further characterized in that the vacuumapproaches 1 mm. of Hg absolute during subjection of the steel to thevacuum.

19. In a process of removing deleterious gases by vacuum treatment froma batch of molten steel, the steps comprising subjecting the surface ofthe batch to a vacuum sufficiently low to effectively degas it, and,simultaneously with subjection of the steel to the vacuum, passing dryair upwardly through the batch to thereby cause substantially undegassedmolten steel in areas of the batch remote from the surface to be movedto the surface.

20. The process of claim 19 further characterized in that the vacuumapproaches 1 mm. of Hg absolute during subjection of the steel to thevacuum. uum treatment from a batch of molten steel, the steps 21. In aprocess of removing deleterious gases by vacuum treatment from a batchof molten steel, the steps comprising subjecting the surface of thebatch to a vacuum sufficiently low to effectively degas it in theabsence of added heat, and, simultaneously with subjection of the steelto the vacuum, passing a gaseous purging agent which does not combinewith or migrate into the steel upwardly through the batch to therebycause substantially undegassed molten steel in areas of the batch remotefrom the surface to be moved to the surface.

22. The process of claim 21 further characterized in that simultaneousvacuum and purging treatment occurs in the absence of a slag blanketwhich prevents the boiling metal from being effectively exposed to thevacuum.

23. The process of claim 22 in which the gaseous purging agent is dryair.

References Cited by the Examiner UNITED STATES PATENTS 1,921,060 8/1933Williams -49 x 2,054,922 9/1936 Betterton et al. 26634 2,054,923 9/1936Betterton et al 75-49 x 2,726,952 12/1955 Morgan 75-49 2,776,204 1/1957Moore 75-49 2,826,489 3/1958 Wagner 75 49 x 2,852,246 9/1958 Janco 266342,871,008 1/1959 Spire 75-60 2,893,860 7/1959 Lorenz 75-49 FOREIGNPATENTS 568,803 7/1958 Belgium.

BENJAMIN HENKIN, Primary Examiner.

RAY K. WINDHAM, DAVID L. RECK, Examiners.

1. A METHOD OF REMOVING DELETERIOUS GASES FROM A CONFINED VOLUME OFMOLTEN METAL IN A RECEPTACLE, SAID METHOD INCLUDING THE STEPS OFSUBJECTING THE SURFACE OF THE CONFINED VOLUME OF MOLTEN METAL TO AVACUUM SUFFICIENT TO DEGAS THE MOLTEN METAL, AND SIMULTANEOUSLY PASSINGA SUFFICIENT QUANTITY OF A CARRIER AGENT UPWARDLY THROUGH THE MOLTENMETAL TO INDUCE A CIRCULATION ENTIRELY WITHIN THE